"People don't need to worry about their Medicare Part B," Wheaton said. "Our system error caused this letter to be sent by the Social Security Administration."

Medicare is a federal program, but the state helps administer it by determining eligibility. Wheaton said the state is working with federal officials, who plan to notify seniors of the error.

The letter led some seniors to believe that their next Social Security check would be reduced by the amount of the subsidy. The assistance can subsidize a Part B premium by up to $1,100 per year, or $91 per month, for seniors with a monthly income below 135% of the poverty line.

"We were able to catch it quick enough so that it didn't actually happen," Wheaton said. "Seniors don't need to do anything. It's being taken care of by our office."

Contact John Wisely: 313-222-6825 or jwisely@freepress.com. On Twitter @jwisely.